In the semiconductor industry, plasmas are widely utilized in the processing of silicon wafers. Plasma chambers are typically used for the deposition and/or etching of material on/from a substrate. Given the dynamic state of plasma, there is a consistent need to detect and control the instantaneous discharge of electrons, known as an arc, between two nodes of differing potential. Arcing is a common problem in plasma processing systems for various reasons. First of all, since it involves rapid discharge, arcing can often be destructive and can destroy and/or wear down parts within the plasma chamber. Also, the presence of arcing can affect various process parameters, such as the deposition and/or etch rates, thereby causing non-uniformities on the processed wafer. Further, arcing can cause defects in the wafer surface, which ultimately reduces the yield of working semiconductor devices fabricated on the wafer. Thus, it is desirable to find an effective method to detect, isolate and prevent arcs from happening in a plasma chamber during wafer processing.
Arcing can be considered a form of instability within the plasma chamber. Since it is known that plasma instabilities can lead to difficulties in process control (which in turn can reduce process repeatability), methods have been developed to minimize plasma instabilities in general.
FIG. 1 is a block diagram of a conventional plasma processing system 100, which employs feedback control to minimize plasma instabilities. System 100 includes a plasma chamber 150, a power generator 110, a power modulator 120 and a signal detector 130.
In operation, power generator 110 directs power (e.g. RF power) to plasma chamber 150 via, for example, an antenna or capacitive-coupling device. The supplied power enables formation of the plasma. Signal detector 130 collects a signal from the plasma that is related to a parameter of the plasma, and can have a particular relationship or correlation to the parameter of the plasma (e.g. electron density, electron temperature, ion density, positive ion temperature). Power modulator 120 is operable to modulate the power produced by power generator 110, in response to the detected signal, to reduce an instability of the parameter of the plasma. In this manner, instabilities in the plasma are minimized by feedback control of the power supplied to plasma chamber 150.
However, this basic system can only control the power supplied to the plasma chamber; it cannot directly control instabilities that may occur inside the plasma chamber. Also, system 100 is mainly geared for minimizing general plasma instabilities, which may or may not be related to arcing. Therefore it is more desirable to employ a method and system that is specifically designed to diagnose arcing in plasma processing chambers.
FIG. 2 shows a flowchart illustrating a conventional method 200 for reducing arcing in a plasma processing chamber. The method 200 may start with the coupling of a voltage probe to the gas distribution faceplate (202) of the processing chamber and the subsequent measurement of the faceplate voltage (204). A high-speed voltage measurement device may be coupled to the voltage probe to generate a plot of the faceplate voltage measurements over time (206). The plot may include features (e.g. voltage spikes) that indicate arcing in the processing chamber, and these features may be used to diagnose and correct the underlying causes of the arcing.
In method 200, three adjustments are made to the plasma deposition process to reduce (or eliminate) arcing during the plasma deposition. These adjustments may include changing the RF power level (208), such as reducing the overall RF power supplied to the processing chamber. When multiple frequencies of RF power are supplied to the processing chamber, the power adjustment may be made to one or more RF frequencies (e.g. adjusting either the LF RF power level or the HF RF power level in a two-frequency RF source). Power level adjustments may also include decreasing or stopping the RF power before the end of the deposition to avoid arcing caused by voltage buildup in the process chamber.
Adjustments may also be made to the ramp rate at which the RF power is supplied to the processing chamber (210). In conventional PECVD deposition processes, the HF RF power is commonly ramped to the peak power level as fast as possible (e.g. 5000 watts/sec or faster) Adjustment to this ramp rate may include lowering the ramp rate for HF RF power and/or the LF RF power, and may also include ramping the power in steps instead of one continuous increase from zero watts to the peak power level.
Adjustments may further be made to the flow rates of one or more of the precursor gases (212) used to form the plasma. For example, in a plasma deposition of a fluorine-doped silicate glass (FSG) film, the flow rate of the silicon or fluorine precursor gas may be reduced to avoid arcing. The adjustments may also include a change in the timing of the introduction of one or more precursors to the processing chamber. For example, the introduction of a fluorine precursor may start before the RF power is activated to reduce arcing during the initial formation of the plasma in the processing chamber.
Depending on the characteristics of the deposition process, any combination of one or more of the adjustments 208, 210, and 212 maybe be implemented in order to reduce or to eliminate arcing.
While method 200 allows for the detection of disturbances seen within the bulk plasma (via observing spikes in plot of faceplate voltage, in step 206), it does not provide a means of feed forward mitigation of the arc (arcs can only be detected once they have happened; any adjustments made are for prevention of future arcs). Furthermore, method 200 does not provide any specific information regarding the arc (the location, duration, intensity, etc).
Other conventional arc detection systems involve monitoring of the power supplied to the plasma chamber and comparing chamber voltages and/or currents to a given threshold. For a given plasma processing system, the power supply to drive the process attempts to regulate power delivered to the chamber. The impedance of the chamber elements, (including the anode, cathode, and chamber environment) is in series with the impedance of the plasma-generating supply circuit. The relation between voltage and current to maintain a constant power in a plasma is dependent upon the impedance of the chamber elements. When an arc develops in a plasma chamber, the magnitude of the impedance of the chamber drops rapidly, thereby changing the impedance of the plasma-generating supply circuit. The power supply and distribution circuit contain significant series inductance, limiting the rate at which current can change in the circuit. A rapid drop in chamber impedance therefore causes a rapid decrease in the magnitude of chamber voltage due to this inductive component. Because the chamber voltage drops rapidly when an arcing event occurs, an unexpected voltage drop below a pre-defined or adaptive voltage threshold level can be used to define the occurrence of an arcing condition. This is the principle behind the conventional system shown in FIG. 3, as will be discussed below.
FIG. 3 illustrates another conventional plasma processing system 300, which employs an arc detection arrangement. Although, system 300 is a physical vapor deposition (PVD) system used for sputtering and deposition, the arc detection arrangement may be implemented in connection with other plasma systems, such as plasma etching systems.
System 300 includes a deposition chamber 310 containing a gas 315, such as argon, at lower pressure. A metal target 320 is placed in vacuum chamber 310 and electrically coupled as a cathode to a power supply 330 via an independent power supply interface module (PSIM) 340. Power supply 330 and chamber 310 are coupled using a coaxial interconnecting cable 335. A substrate (wafer) 325 is coupled as an anode to power supply 330 through a ground connection. Vacuum chamber 310 is also typically coupled to ground. A rotating magnet 327 is included to steer the plasma to maintain uniform target wear. PSIM 340 includes a buffered voltage attenuator 344 adapted to sense the chamber voltage and provide an analog signal to an Arc Detection Unit (ADU) 350 via a voltage signal path 342 responsive to the chamber voltage. PSIM 340 also includes a Hall effect-based current sensor 346 adapted to sense the current flowing to chamber 310 and provide an analog signal via a current signal path 348 to the ADU input responsive to the chamber current. ADU 350 is communicatively coupled to a logic arrangement 360, for example a programmable logic controller (PLC) via a local data interface 370. Logic arrangement 360 may be coupled to a data network 380, for example a high level process control network.
In operation, an electric field is generated between the target 320 (cathode) and substrate 325 (anode) by power supply 330 causing the gas in vacuum chamber 310 to ionize. Ionized gas atoms (e.g. plasma) are accelerated by electric field and impact the target at high speed, causing molecules of the target material to be physically separated from the target, or “sputtered”. The ejected molecules travel virtually unimpeded through the low pressure gas and plasma striking the substrate and forming a coating of target material on substrate 325.
Via voltage signal path 342, ADU 350 monitors the voltage of chamber 310 and detects an arcing condition whenever the voltage magnitude drops below a preset arc threshold voltage value. Also, via current signal path 348, the current flowing, to chamber 310 is monitored and used in detecting arcing events, an arcing event being determined whenever the current magnitude exceeds a preset current threshold value. Threshold values are established by logic arrangement 360. ADU 350 may also be operable to count arcing conditions (events) responsive to at least one threshold. A rate of detected arcing donation occurrences may be determined therefrom. ADU 350 may also contain a clock and a digital counter in order to measure arc duration. In this manner, the quantity and severity (occurrences, duration, intensity, etc.) of arcing, in chamber 310 may be readily assessed, therefore allowing for an accurate estimate of possible damage to processed wafers.
Despite being able to closely monitor arcing, system 300 does not provide visibility into the arc location, and can only mitigate the effect of the arc after it has occurred. Since arcing often introduces defects/non uniformities in processed wafers, it is desirable to have a plasma processing system that is able to prevent arcs from occurring.
What is needed is a plasma processing system that is able to detect, isolate and/or prevent arcing inside the plasma chamber.